If the message "Unable to connect to server" appears when attempting to access a website, it means there is no Internet connection. Data cannot be transferred between the client and host. The client is the computer sending the request for information, and the host is the source of the information.
Every website is composed of files and data stored on a server that, when working properly, has a constant connection to the Internet. Several things can cause the interruption of data flow: a cable could be unplugged or there could be wireless signal interference, the server or client may not be able to resolve IP addresses, or the request could have timed out. Data is separated into packets, converted into raw signals and physically transferred to the destination, which requires a connection. A request that times out simply has taken too long to start and is aborted.
An IP address provides every computer and website home page with a unique address. If the computer cannot convert the domain name to an IP address or vice versa, it cannot complete a data request. This error occurs server-side as well. If the error is server-side, the only thing to do is wait for it to be resolved.